1. Field of the Invention
This invention concerns phosphate mold materials suitable for the casting of glass ceramics with a melting point of 1000.degree. C. or more. 2. Description of the Related Art
Since phosphate mold materials have excellent heat resistance to gypsum mold materials, they are used for base metal alloys, such as Ni-Cr and Co-Cr, which have high melting points. Their usefulness as mold materials is particularly appreciated in the field of dental casting using base metal alloys.
Phosphate mold materials comprise mainly silica (consisting of cristobalite and quartz with different crystal systems), which functions as a refractory material, and phosphates and metal oxides, which function as binders. The phosphate used as a binder may be an acid phosphate, such as primary ammonium phosphate or primary magnesium phosphate. The metal oxide used as a binder may be magnesium oxide, and in particular hard-baked or electromelted magnesia, which has a periclase crystal structure.
The composition of conventional phosphate mold materials is 80-90 weight % of refractories, 10-20 weight % of binders, and minute amounts of other components, which are added to control hardening time, or to adjust gas permeability and to improve storage stability.
Mold materials with these compositions are ground together with 5-20% colloidal silica solutions in order to prepare a mold. After the cast is poured into the mold, the mold is then shattered and the cast is removed.
With alloys having high melting points, however, about 2.0-2.3% shrinkage occurs in the cooling process after pouring, and it is necessary to compensate for this shrinkage if a high precision cast is to be manufactured.
A mold material which permits control the expansion coefficient is disclosed in Unexamined Published Japanese Patent No. 57-9554. The mold material in this disclosure consists of quartz 65 weight %, cristobalite 20 weight %, magnesium oxide 5.6 weight % and primary ammonium phosphate 9.4 weight %, and carboxylic acids are also added.
Conventional mold materials however have a compressive strength of 76-185 Kg/cm.sup.2, and they are thus very hard. If it was attempted to remove a cast of a brittle material, such as glass from the mold, therefore, there was a strong possibility that the cast would also break when the mold was shattered, and removal was therefore difficult.
If the amount of cristobalite in the mold material is increased, the overall expansion can be compensated, but cracks then occur more easily in the mold due to thermal shock.